Hardness tells you how hard one substance is from another. The Janka hardness test measures the hardness of wood. (The Janka rating for non-wooden bones is estimated). If you want sharper tones, then get harder bones. If you want duller tones, then get softer bones.

Get the Tone You Want
What is the difference between a dull tone and a sharp tone? It's just the difference in tone relative to hardness. Scroll down to view musical bones grouped by similar tone quality.

What is the difference between hardness and density?

"Hardness refers to the pressure required to compress the material. Hardness is that property by which a body resists any foreign substance that attempts to force a passage between its particles. The hardness of a body depends on the degree of firmness with which its particles cohere. It is therefore entirely distinct from density, which depends on the number of particles in a given bulk. Thus lead is dense, but not hard."George P. Quackenbos (1826-1881)